1. Field of the Invention
This invention relates to a wide-angle objective lens for cameras, and particularly to a compact, large aperture, wide-angle objective lens suitable for use in a 35 mm format lens shutter camera or a 35 mm format camera with a range finder.
2. Related Background Art
Biogon type, abiogon type, etc. are known as symmetrical type wide-angle lenses having a negative, positive, negative refractive power distribution. The biogon type wide-angle lens can cover a wide angle of view and can make distortion small. Also, its refractive power distribution is a negative, positive, negative refractive power distribution and therefore, as compared with a symmetrical type wide-angle lens of a positive, negative, positive refractive power distribution such as a topogon type wide-angle lens, the biogon type wide-angle lens has an advantage that the quantity of marginal light is great and moreover the diameters of fore and rear lenses can be made small.
Various wide-angle lenses of negative, positive, negative construction are known as the developed types of the biogon type wide-angle lens. They are known, for example, from Japanese Utility Model Publication No. 43-30782, U.S. Pat. No. 3,829,198, U.S. Pat. No. 4,211,472, Japanese Laid-Open Patent Application No. 56-140311, etc.
Examples in which the F-number is made small are shown in Japanese Utility Model Publication No. 43-30782 and U.S. Pat. No. 4,211,472 and examples which comprise a small number of lens components are shown in U.S. Pat. No. 3,829,198 and Japanese Laid-Open Patent Application No. 56-140311.
However, the biogon type wide-angle lens generally suffers from the disadvantages that the total thickness of the lens system (the thickness from that surface of the lens which is most adjacent (nearest) to the object side to that surface of the lens which is most adjacent to the image side) is great and that the F-number is large.
In the lens systems shown in U.S. Pat. No. 3,829,198 and Japanese Laid-Open Patent Application No. 56-140311, the number of lens components is small but the correction of spherical aberration is deficient and therefore, the F-number cannot provide a large aperture. They also have suffered from the disadvantages that the total thickness of the lens system is very great and that the spacing between the front unit and the rear unit of the lens system in which a diaphragm is placed is too narrow and therefore great limitations must be imposed upon the structure of an aperture stop, a shutter unit, a lens barrel, etc.
In the lens system shown in Japanese Utility Model Publication No. 43-30782, the total thickness of the lens system is very great. Moreover the diameters of the fore and rear lenses are great. This is against the desire to make the lens system compact. In the lens system shown in U.S. Pat. No. 4,211,472, the F-number provides a large aperture but the total thickness of the lens system is great, and this is against the desire to make the lens compact. Also, the shape of coma is bad and therefore, if the lens system is left in its original condition, it is necessary to apply a limitation to the light beam and the quantity of marginal light becomes very small, and this has not been preferable.
Further, in the lens system of Japanese Laid-Open Patent Application No. 54-70826, the symmetry of the refractive power distributions on both sides of a stop is destroyed, the air space between a negative lens component disposed on the object side and a positive lens component and the air space between the positive lens component and a negative lens component disposed on the image side are both are widened, and a wide-angle of view and a large aperture can be obtained. However, this lens system has the disadvantages that the great air spaces result in a great total thickness of the lens system and that the height of a ray of light passing through the lens component remote from the stop becomes far from the optical axis and therefore the effective diameter of each lens becomes large.
Further, Gaussian type, Topogon type, etc. are known as symmetrical type lenses having positive, negative, positive refractive power distribution.
The positive, negative, positive symmetrical type lens is advantageous for large aperture, but cannot cover a wide-angle of view, and conversely, the aforedescribed negative, positive, negative symmetrical lens can cover a wide-angle of view, but has the disadvantage for large aperture.
In the positive, negative, positive symmetrical type lens, the off-axis light beam passing through the lens component remote from the stop passes a location remote from the optical axis and therefore, when an attempt is made to achieve a wide-angle, the effective diameters of the front and rear lenses become large, and this results in the bulkiness of the optical system. Accordingly, when an attempt is made to achieve a wide-angle, a negative, positive, negative symmetrical type lens is desirable.
Thus, it has been difficult to provide an optical system which satisfies the condition that a wide-angle can be achieved by a symmetrical type lens and the optical system is compact with a large aperture. The positive, negative, positive symmetrical type lens and the negative, positive, negative symmetrical type lens are generally of a construction advantageous in the correction of distortion and chromatic aberration. When in such symmetrical type lenses, the whole lens system is axially moved to effect focusing (so-called whole axial movement system), the fluctuation of off-axis aberration can be suppressed to a certain degree.
However, where the whole axial movement system is used in the optical system as disclosed in Japanese Laid-Open Patent Application No. 54-170826 wherein the symmetry of the refractive power arrangement of the negative, positive, negative symmetrical type lens is greatly destroyed in order to make the optical system bright, there has been a problem that the fluctuation of off-axis aberration becomes great.